The Applicant of the present invention discloses a magnetostrictive torque sensor that contactlessly detects a rotational torque about an axis acting on a rotary shaft, for example, in Japanese Patent Application Publication No. 2004-239652. The magnetostrictive torque sensor according to Japanese Patent Application Publication No. 2004-239652 has its rotary shaft formed of a chromium molybdenum steel material (hereinafter abbreviated as “SCM material” in some cases). A magnetostrictive film is attached in advance to a rotary shaft made of an SCM material, and then heat treatment such as carburizing treatment is applied to the SCM material to set the Rockwell hardness of the rotary shaft within a range of 40 to 65 HRC.
The magnetostrictive torque sensor according to Japanese Patent Application Publication No. 2004-239652 takes an approach of increasing the hardness of the rotary shaft for suppressing the rotary shaft from having large plastic deformation or the magnetostrictive film from being peeled off, even when an excessive torque acts on the rotary shaft, to suitably keep the detection accuracy.